Pulse oximeters typically measure and display various blood flow characteristics including but not limited to the oxygen saturation of hemoglobin in arterial blood. Oximeters pass light through blood perfused tissue such as a finger or an ear, and photoelectrically sense the absorption of light in the tissue. The amount of light absorbed is then used to calculate the amount of the blood constituent (e.g., oxyliemoglobin) being measured.
The light passed through the tissue is selected to be of one or more wavelengths that are absorbed by the blood in an amount representative of the amount of the blood constituent present in the blood. The amount of light passed through the tissue varies in accordance with the changing amount of blood constituent in the tissue and the related light absorption.
The optical signal through the tissue may be degraded by noise or other artifacts. One source of noise is ambient light which reaches the light detector. Another source of noise is electromagnetic coupling from other electronic instruments. Motion of the patient may also introduce noise onto the signal. For example, the contact between the detector and the skin, or the emitter and the skin, may be temporarily disrupted when motion causes either to move away from the skin. In addition, since blood is a fluid, it may respond differently than the surrounding tissue to inertial effects, thus resulting in momentary changes in volume at the point to which the oximeter probe is attached. Noise may degrade a pulse oximetry signal relied upon by a physician, without the physicians awareness.
An oximetry algorithm may include noise metrics to allow it to quantify artifacts that degrade SpO2 accuracy. These metrics may be used to reduce the impact of the artifacts on accuracy. For example, these metrics may be used to adapt multiple internal filters and to select from multiple internal SpO2 estimates. In cases with motion artifact, the noise metrics generally have a significant negative correlation to the SpO2 level for typical sensor designs. That is, increased motion artifact usually results in a negative SpO2 bias for transmission-type sensor designs, rather than simply generating zero-mean SpO2 errors.